1. Field of the Invention
The present invention relates in general to an exhaust manifold for a multicylinder internal combustion engine, and more particularly to an exhaust manifold which is designed to exhibit excellent durability against heat applied thereto.
2. Description of the Prior Art
Hitherto, various types of exhaust manifolds for a multicylinder type internal combustion engine have been proposed and put into practical use. One of them is disclosed in Japanese Utility Model First Provisional Publication No. 60-3225.
The exhaust manifold proposed by the Publication is shown in FIGS. 7 and 8 of the attached drawings, as being generally designated by reference numeral 1. As is seen from FIG. 7, the manifold 1 comprises generally a plurality of branch runners 4 each having an upstream end connected through a flange 3 to an exhaust port 2 formed in a cylinder head (not shown) of the engine. The two outside branch runners 4 are united at their downstream portions to form a leftside united hollow portion 5a, while the other two inside branch runners 4 are also united at their downstream portions to form a rightside united hollow portion 5b which is arranged beside the leftside united hollow portion 5a. These two united hollow portions 5a and 5b are connected at their downstream ends to a common flange 6 by welding. In order to install an exhaust gas sensor 7 or the like to the exhaust manifold 1, there is provided a sensor mount 8 in the form of a collar at the position where the inside walls of the two united hollow portions 5a and 5b are attached to each other. As is seen from FIG. 8, the sensor 7 is tightly disposed on the sensor mount 8 having its probe portion 7a exposed to the interior of the united hollow portions 5a and 5b.
However, the exhaust manifold 1 has some drawbacks which will be described in the following.
Usually, during operation of the engine, the united hollow portions 5a and 5b are heated up to a temperature of about 800.degree. C. to 900.degree. C. because of poor heat radiation appearing at these portions. This causes reduction in mechanical strength of the united hollow portions 5a and 5b. In fact, heating these portions to such high temperature brings about elongation of the branch runners 4 generating a remarkable thermal stress at the welded portion between the runners 4 and the common flange 6 as well as the welded portion between the runners 4 and the sensor mount 8. Thus, it sometimes occurs that the welded portions are broken and the common flange 6 is deformed causing a leakage of exhaust gas through a crack formed in the welded portions.
Furthermore, preparation of such a separate common flange 6 and a separate sensor mount 8 which are to be welded to the united hollow portions 5a and 5b thereafter brings about increase in production cost of the exhaust manifold 1.